Meltblown absorbent fibers and composites and method for making the same

ABSTRACT

An absorbent fiber is produced from a melt processable polymer. An absorbent composite includes the absorbent fiber in addition to natural fibers and superabsorbent material. A method of producing the superabsorbent fiber and absorbent composite is also disclosed.

FIELD OF THE INVENTION

[0001] This invention relates to absorbent materials having improvedabsorbent properties. More specifically, this invention relates toabsorbent fibers produced from a melt processable polymer and toabsorbent composites containing the absorbent fibers. This inventionalso relates to a method for making the absorbent fibers and absorbentcomposites.

BACKGROUND OF THE INVENTION

[0002] Absorbent materials are useful in disposable personal careabsorbent products such as diapers, training pants, feminine pads, adultincontinence products and professional health care products forabsorbing and retaining fluids. Absorbent or superabsorbent materialsare often combined with water-insoluble fibers to create an absorbentcomposite for use in an absorbent core of a disposable personal careabsorbent product according to methods known in the art.

[0003] Particulate superabsorbents are widely used as superabsorbentmaterial in disposable personal care absorbent products. However,superabsorbent particles are sometimes difficult to use because they donot remain stationary during the manufacturing of the disposablepersonal care absorbent product and may shift position in the disposablepersonal care absorbent product.

[0004] The use of absorbent or superabsorbent fibers instead ofsuperabsorbent particles in disposable personal care absorbent productsis potentially advantageous because fibers can provide improved productintegrity, better containment, reduced product bulkiness and improvedabsorbent properties, such as rapid fluid absorption and fluiddistribution properties. Furthermore, the use of fibers may also lead toimproved product attributes, such as thinner and softer products thatprovide better fit, less gel migration, and potential simplification ofthe manufacturing process of the absorbent product.

[0005] Currently available commercial absorbent fibers are produced bysolution spinning processes, such as a solution dry spinning processused for textile fiber manufacturing or a solution blowing spinningprocess used for nonwoven fabric manufacturing. Solution spinningprocesses start with an aqueous polymer solution. The polymer solutionis then spun into fibers and cross-linked to form water-swellable,water-insoluble fibers. However, such solution spinning processes havelow productivity and are expensive because of the presence of excesswater in the polymer solution. For these reasons, absorbent fibers arenot widely used for the absorbent material in disposable personal careabsorbent products.

[0006] Some polymers can be processed into different shapes or formswhen the temperature is above a certain point and can be referred to as“melt processable.” Other polymers upon reaching a certain elevatedtemperature will degrade, rather than melt, and can be referred to as“non-melt processable.”

[0007] Fibers produced from melt processable polymers such aspolyethylene (PE) or polypropylene (PP) are widely used in nonwovenindustries at low cost. However, they cannot be used in absorbentcomposites of disposable personal care absorbent products due to theirhigh hydrophobicity which causes poor fluid handling properties such asnon-wetting and no wicking.

[0008] U.S. Pat. No. 5,280,079 issued Jan. 18, 1994 to Allen et al.,U.S. Pat. No. 5,147,956 issued Sep. 15, 1992 to Allen, U.S. Pat. No.4,962,172 issued Oct. 09, 1990 to Allen et al., U.S. Pat. No. 5,151,465issued Sep. 29, 1992 to Le-Khac, and U.S. Pat. No. 5,066,742 issued Nov.19, 1991 to Gupta each describe absorbent fibers. However, the absorbentfibers described therein are produced by a solution spinning process. Inaddition, the water soluble polymers used to form the fibers arenon-melt processable. Furthermore, superabsorbent staple fibers arecommerically available from Camelot Superabsorbent Ltd. in Calgary,Canada under the trade designation FIBERDRI®, an isobutylene-maleicanhydride copolymer based absorbent fiber, or from Technical Absorbentsin Grimsby, United Kingdom under the trade designation OASIS® 101, asodium polyacrylate based absorbent fiber. These commerically availablesuperabsorbent fibers are also produced by solution spinning of non-meltprocessable water soluble polymers. These staple fibers are made fromtextile fiber dry spinning process. They can be incorporated intoabsorbent cores through an air laying process but do not form bonds withother components of the core, such as superabsorbent particles and flufffiber. The absorbent core from the process is not a stabilizedstructure.

[0009] There is a need for absorbent fibers that can be manufactured inhigh productivity and at low cost, and for absorbent compositescontaining such absorbent fibers. There is also a need for stabilizedabsorbent composites that can exhibit improved dry and wet integrities.There is also a need for a high productivity, low cost method for makingthe absorbent fibers and absorbent composites.

SUMMARY OF THE INVENTION

[0010] This invention relates to absorbent fibers produced from meltprocessable polymers and to absorbent composites containing theabsorbent fibers. This invention also relates to a method for making theabsorbent fibers and absorbent composites.

[0011] In one embodiment of this invention, the absorbent fibers includea melt processable, water soluble polymer which is meltblown and thencross-linked to form the water-swellable but water insoluble absorbentfibers. The resulting absorbent fibers have an absorbency under zeroload value of at least about 5 grams fluid per gram fiber (g/g). Themelt processable, water soluble polymer may be a non-ionic homopolymer,such as for example, polyethylene oxide, polypropylene oxide, hydroxypropyl cellulose, methyl cellulose, ethyl cellulose, methyethylcellulose, polyethylene imine, polyvinyl amine, polyvinyl alcohol,poly(ethylene oxide-co-propylene oxide), polyacrylic acid,polyacrylamide, and combinations thereof. The melt processable, watersoluble polymer may also be a copolymer of monomers of at least oneionic and one non-ionic monomer such as sodium acrylate (currently usedin commercial superabsorbent materials) and methyl methacrylate(currently used in commercial melt processable polymers).

[0012] In another embodiment of this invention, an absorbent compositeincludes a melt processable, water soluble polymer which is meltblownwith hydrophilic fibers (such as wood pulp fluff, cotton, cotton linter,other cellulose fibers, regenerated cellulose fibers, natural fibers ormodified or spun staple fibers, and hydrophilic synthetic fibers, suchas those available from Allied Corporation in Morristown, N.J., USA,under the trade designation HYDROFIL®, and combinations thereof) andcommercially available superabsorbent material. The polymer iscross-linked to form water-swellable but water insoluble absorbentfibers. The resulting absorbent composite has an absorbency under zeroload value of at least 5 grams fluid per gram composite (g/g) and mayalso be superabsorbent, exhibiting an absorbency under zero load of atleast about 10 g/g or up to about 50 g/g. As in the previous embodiment,the melt processable, water soluble polymer may be a non-ionichomopolymer, such as for example, polyethylene oxide, polypropyleneoxide, hydroxy propyl cellulose, methyl cellulose, ethyl cellulose,methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinylalcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid,polyacrylamide, and combinations thereof, or may be a copolymer ofmonomers of at least one ionic and one non-ionic monomer such as sodiumacrylate and methyl methacrylate.

[0013] In either embodiment, the cross-linking agent can be sprayed ontothe surface of the meltblown fibers. The cross-linking agent must haveat least two functional groups capable of reacting with the functionalgroups on the surface of the melt processable polymer. In order toinitiate the cross-linking reaction, a post treatment such asheat,treatment, microwave radiation, electron beam (e-beam) radiation,ultraviolet (UV) radiation, steam treatment or vapor treatment isrequired.

[0014] This invention also relates to a method for making absorbentfibers and absorbent composites including the steps of melting a meltprocessable, water soluble polymer, extruding the polymer, spinning thepolymer to form fibers, adding a cross-linking agent and curing theresulting fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is an exploded perspective view of a diaper with anabsorbent core containing absorbent material FIGS. 2A-2C showphotographs of an absorbent composite according to one embodiment of theinvention.

[0016]FIG. 3 is a schematic representation of a method and apparatus forproducing absorbent fibers and absorbent composites according to oneembodiment of the invention.

DEFINITIONS

[0017] Within the context of this specification, each term or phrasebelow will include the following meaning or meanings.

[0018] “Polymer” includes, but is not limited to, homopolymers,copolymers, such as for example, block, graft, random and alternatingcopolymers, terpolymers, etc., and blends and modifications thereof.Furthermore, unless otherwise specifically limited, the term “polymer”shall include all possible geometrical configurations of the material.These configurations include, but are not limited to isotactic,syndiotactic and atactic symmetries.

[0019] “Nonwoven fabric or web” means a web having a structure ofindividual fibers or threads which are interlaid, but not in a regularor identifiable manner as in a knitted fabric. Nonwoven fabrics or webshave been formed from many processes such as, for example, meltblowingprocesses, spunbonding processes, air laying processes, and bondedcarded web processes. The basis weight of nonwoven fabrics is usuallyexpressed in ounces of material per square yard (osy) or grams persquare meter (gsm) and the fiber diameters useful are usually expressedin microns. (Note that to convert from osy to gsm, multiply osy by33.91.).

[0020] “Spunbonded fibers” refers to small diameter fibers which areformed by extruding molten thermoplastic material as filaments from aplurality of fine capillaries of a spinnerette having a circular orother configuration, with the diameter of the extruded filaments thenbeing rapidly reduced as by, for example, in U.S. Pat. No. 4,340,563 toAppel et al., and U.S. Pat. No. 3,692,618 to Dorschner et al., U.S. Pat.No. 3,802,817 to Matsuki et al., U.S. Pat. Nos. 3,338,992 and 3,341,394to Kinney, U.S. Pat. No. 3,502,763 to Hartmann, U.S. Pat. No. 3,502,538to Petersen, and U.S. Pat. No. 3,542,615 to Dobo et al., each of whichis incorporated herein in its entirety by reference. Spunbond fibers arequenched and generally not tacky when they are deposited onto acollecting surface. Spunbond fibers are generally continuous and oftenhave average diameters larger than about 7 microns, more particularly,between about 10 and 30 microns.

[0021] “Meltblown fibers” means fibers formed by extruding a moltenthermoplastic material through a plurality of fine, usually circular,die capillaries as molten threads or filaments into converging highvelocity heated gas (e.g., air) streams which attenuate the filaments ofmolten thermoplastic material to reduce their diameter, which may be tomicrofiber diameter. Thereafter, the meltblown fibers are carried by thehigh velocity gas stream and are deposited on a collecting surface toform a web of randomly dispersed meltblown fibers. Such a process isdisclosed for example, in U.S. Pat. No. 3,849,241 to Butin et al.Meltblown fibers are microfibers which may be continuous ordiscontinuous, are generally smaller than 10 microns in diameter, andare generally self bonding when deposited onto a collecting surface.Meltblown fibers used in the present invention are preferablysubstantially continuous in length.

[0022] “Coform material” refers to a product produced by combiningseparate polymer and additive streams into a single deposition stream informing the nonwoven webs. Such a process is taught, for example, byU.S. Pat. No. 4,100,324 to Anderson et al. which is hereby incorporatedby reference. U.S. Pat. No. 4,818,464 to Lau discloses the introductionof superabsorbent material as well as wood pulp fluff, cellulose, orstaple fibers through a centralized chute in an extrusion die forcombination with resin fibers in a nonwoven web. The wood pulp fluff,staple fibers, or other material are added to vary the characteristicsof the resulting web, for example, strength and absorbency.

[0023] “Pulp fibers” refers to fibers from natural sources such as woodyand non-woody plants. Woody plants include, for example, deciduous andconiferous trees. Non-woody plants include, for instance, cotton, flax,esparto grass, milkweed, straw, jute hemp, and bagasse.

[0024] “Cross-linked” refers to any means for effectively renderingnormally water-soluble materials substantially water insoluble butswellable. Such means can include, for example, physical entanglement,crystalline domains, covalent bonds, ionic complexes and associations,hydrophilic associations, such as hydrogen bonding, and hydrophobicassociations or Van der Waals forces.

[0025] “Hydrophilic” describes fibers or the surfaces of fibers whichare wettable by the aqueous liquids in contact with the fibers. Thedegree of wetting of the materials can, in turn, be described in termsof the contact angles and the surface tensions of the liquids andmaterials involved. Equipment and techniques suitable for measuring thewettability of particular fiber materials or blends of fiber materialscan be provided by a Cahn SFA-222 Surface Force Analyzer System, or asubstantially equivalent system. When measured with this system, fibershaving contact angles less than 90° are designated “wettable” orhydrophilic, while fibers having contact angles greater than 90° aredesignated “nonwettable” or hydrophobic.

[0026] “Superabsorbent material” refers to a water-swellable,water-insoluble organic or inorganic material capable, under the mostfavorable conditions, of absorbing at least about 10 times its weight,preferably at least about 20 times its weight in an aqueous solutioncontaining 0.9% by weight sodium chloride. Superabsorbent material cancomprise a form including particles, fibers, nonwovens, films, coforms,printings, coatings, other structural forms, and combinations thereof.“Water-swellable, water-insoluble” refers to the ability of a materialto swell to a equilibrium volume in excess water but not dissolve intothe water. The water-swellable, water-insoluble material generallyretains its original identity or physical structure, but in a highlyexpanded state upon the absorption of water.

[0027] “Absorbency Under Zero Load (AUZL)” refers to the result of atest which measures the amount in grams of an aqueous 0.9% by weightsodium chloride solution that a gram of material can absorb in 1 hourunder negligible applied load (about 0.01 pound per square inch).

[0028] “Water-soluble” refers to materials which substantially dissolvein excess water to form a solution, thereby losing its initial form andbecoming essentially molecularly dispersed throughout the watersolution. As a general rule, a water-soluble material will be free froma substantial degree of cross-linking, as cross-linking tends to rendera material water insoluble. A material that is “water insoluble” is onethat is not water soluble according to this definition.

[0029] “Melt processable” refers to either a crystalline orsemicrystalline polymer that has a melting point or an amorphous polymerthat has a softening point and, therefore, can be thermally processedinto different shapes or forms, for example, meltblown fibers. In orderto be considered as a melt processable polymer, the crystalline orsemicrystalline polymers have to have a melting point as well asreasonable thermal stability and melt processability such as adequatemelt rheology. For amorphous polymers, they have to have a softeningpoint as well as reasonable thermal stability and melt processabilitysuch as adequate melt rheology.

[0030] “Solvent” refers to a substance, particularly in liquid form,that is capable of dissolving a polymer used herein to form asubstantially uniformly dispersed mixture at the molecular level.

[0031] The term “absorbent product” includes without limitation diapers,training pants, swim wear, absorbent underpants, baby wipes, adultincontinence products, feminine hygiene products, medical garments,underpads, bandages, absorbent drapes, and medical wipes, as well asindustrial work wear garments.

[0032] These terms may be defined with additional language in theremaining portions of the specification.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0033] This invention relates to absorbent fibers produced from a meltprocessable polymer and to absorbent composites containing the absorbentfibers. The absorbent composites can be used in absorbent cores fordisposable personal care absorbent products. The absorbent compositesare useful in absorbent articles such as diapers, training pants, swimwear, adult incontinence articles, feminine care products, and medicalabsorbent products. This invention also relates to a method for makingthe absorbent fibers and absorbent composites.

[0034]FIG. 1 illustrates an exploded perspective view of a disposablediaper. Referring to FIG. 1, disposable diaper 10 includes outer cover12, body-side liner 14, and absorbent core 40 located between body-sideliner 14 and outer cover 12. Absorbent core 40 can include the absorbentfibers or an absorbent composite according to this invention. Body-sideliner 14 and outer cover 12 are constructed of conventionalnon-absorbent materials. By “non-absorbent” it is meant that thesematerials, excluding the pockets filled with superabsorbent, have anabsorptive capacity not exceeding 5 grams of 0.9% aqueous sodiumchloride solution per gram of material.

[0035] Body-side liner 14 is constructed from highly liquid perviousmaterials. This layer functions to transfer liquid from the wearer toabsorbent core 40. Suitable liquid pervious materials include porouswoven materials, porous nonwoven materials, films with apertures,open-celled foams, and batting. Other examples of suitable body-sideliner materials include, without limitation, any flexible porous sheetsof polyolefin fibers, such as polypropylene, polyethylene or polyesterfibers; webs of spunbonded polypropylene, polyethylene or polyesterfibers; webs of rayon fibers; bonded carded webs of synthetic or naturalfibers or combinations thereof. U.S. Pat. No. 5,904,675, issued May 18,1999 to Laux et al. and incorporated by reference, provides furtherexamples of suitable surge materials. This layer may also be anapertured plastic film. Suitable batting includes certain air formedthermochemical and chemithermomechanical wood pulps. The various layersof article 10 have dimensions which vary depending on the size and shapeof the wearer.

[0036] Outer cover material 12 should be breathable to water vapor.Generally outer cover 12 will have a moisture vapor transmission rate(MVTR) of at least about 300 grams/m²-24 hours, desirably at least about1000 grams/m²-24 hours, or at least about 3000 grams/m²-24 hours,measured using INDA Test Method IST-70.4-99, herein incorporated byreference.

[0037] Attached to outer cover 12 are waist elastics 26, fastening tapes28 and leg elastics 30. The leg elastics 30 typically have a carriersheet 32 and individual elastic strands 34. The diaper of FIG. 1 is ageneral representation of one basic diaper embodiment. Variousmodifications can be made to the design and materials of diaper parts.

[0038] Construction methods and materials of an embodiment of a diapersuch as illustrated in FIG. 1, are set forth in greater detail incommonly assigned U.S. Pat. No. 5,509,915, issued Apr. 23, 1996 in thename of Hanson et al., incorporated herein by reference. Possiblemodifications to the diaper illustrated in FIG. 1 are set forth incommonly assigned U.S. Pat. No. 5,509,915 and in commonly assigned U.S.Pat. No. 5,364,382, issued Nov. 15, 1994 to Latimer et al.

[0039] According to one embodiment of this invention, the absorbentfibers comprise a melt processable, water soluble polymer which ismeltblown and then cross-linked to form water-swellable but waterinsoluble absorbent fibers. Suitable melt processable, water solublepolymers include non-ionic homopolymers such as polyethylene oxide,polypropylene oxide, hydroxyl propyl cellulose, methyl cellulose, ethylcellulose, methylethyl cellulose, polyethylene imine, polyvinyl amine,polyvinyl alcohol, poly(ethylene oxide-co-propylene oxide), polyacrylicacid, polyacrylamide and combinations of the foregoing. In order toenhance their melt processabilities, some degree of modification may beneeded. These modifications include, but are not limited to, additionsof low percentage of additives, blends, and/or comonomers. The modifiedpolyvinyl alcohol used herein is available commercially from NipponGohsei located in Osaka, Japan.

[0040] Although a non-ionic water soluble and melt processable polymeris absorbent, it is not superabsorbent due to lack of ionic chargegroups on its macromolecular chains. Current commercial particulatesuperabsorbent materials are made of ionic polyacrylate. However, pureionic water soluble polymers in general are not melt processable.

[0041] Another melt processable, water soluble polymer may be acopolymer of both ionic and non-ionic monomers. Suitable monomers forthe copolymer include an ionic monomer, such as sodium acrylate, whichis commerically available from Aldrich Chemical Co. in Milwaukee, Wis.,USA, and a non-ionic monomer, such as methyl methacrylate, which iscommerically available from Aldrich Chemical Co. in Milwaukee, Wis.,USA. In order to achieve both water solubility and melt processabilityof the copolymer, the ratio of the monomers on a dry weight basis iscritical. Preferably, the ratio of the monomers on a dry weight basisshould be from about 30:70 to about 70:30. Polymerization to form thecopolymer can be carried out according to conventional methods known inthe art. Because of the addition of an ionic comonomer, the watersoluble and melt processable copolymers have a higher absorbency thanthe non-ionic homopolymers.

[0042] Whether a homopolymer or a copolymer, the molecular weight of thepolymer is important. The molecular weight of the polymer must be atleast about 10,000 in order to have a high fluid absorbency. However,the molecular weight of the polymer cannot exceed about 1,000,000because the meltblowing equipment can not handle too high of aviscosity. Suitable polymers have a molecular weight between about50,000 to 1,000,000, desirably between about 100,000 to 1,000,000, orbetween about 100,000 and 500,000.

[0043] Following formation of the fibers, the fibers are still watersoluble. A solution containing a cross-linking agent is sprayed onto thesurface of the meltblown fibers to form water-swellable, but waterinsoluble fibers instantly or after the curing step depending on thenature of the cross-linking agent used. The suitable cross-linking agentcan be either reactive or latent. The reactive cross-linking agent willcross-link the fibers in the spinning process. The latent cross-linkingagent does not cross-link the fibers and normally requires someactivation energy to trigger cross-linking, such as heating. Thecross-linking agent desirably has at least two functional groups capableof reacting with the pendant functional groups on the melt processablepolymer. Suitable cross-linking agents include diols, polyols, diamines,polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes,polyaldehydes, butandiol, diethylene triamine, citric acid, glutaricdialdehyde and ethylene glycol diglycidyl ether, tri-valent ortetra-valent metal ions, and combinations of the foregoing.

[0044] The appropriate functional groups on the cross-linking agentdepends upon the melt processable polymer. For example, if polyvinylalcohol is used as the melt processable polymer, suitable functionalgroups on the cross-linking agent include carboxylic acid groups(forming ester linkages with the hydroxyl groups on the polyvinylalcohol), aldehyde groups (forming acetal linkages with the hydroxylgroups on the polyvinyl alcohol), or epoxy groups (forming etherlinkages with the hydroxyl groups on the polyvinyl alcohol). However, ifthe melt processable polymers have different types of functional groups,such as amino, or carboxylic acid, or others, the appropriate functionalgroups on the cross-linking agent will be different. For example, if thepolymer has carboxylic acid functional groups, suitable functionalgroups on the cross-linking agent include hydroxyl groups (forming esterlinkages with the carboxylic acid groups on the polymer), amino groups(forming amide linkages with the carboxylic acid groups on the polymer),or tri-valent or tetra-valent metal ions (forming ionic bonds with thecarboxylic acid groups on the polymer).

[0045] One example of a commercially available cross-linking agent isKYMENE® available from Hercules Incorporated located in Wilmington, Del.KYMENE® contains functional groups which are capable of reacting withhydroxyl groups on polyvinyl alcohol. KYMENE® is widely used tocross-link cellulose fibers. However, the chemical composition isproprietary.

[0046] When a latent cross-linking agent is used, in order to initiatethe cross-linking reaction, a posttreatment process is required. Suchpost treatment processes include heat treatment, microwave radiation,e-beam radiation, UV radiation, steam treatment or vapor treatment.

[0047] According to another embodiment of this invention, the absorbentcomposite includes a melt processable, water soluble polymer which ismeltblown with hydrophilic fibers and commercially availablesuperabsorbent material to form a coform material. The polymer iscross-linked to form water-swellable but water insoluble absorbentfibers.

[0048] Any of the previously described homopolymers or copolymers with amolecular weight between about 10,000 to 1,000,000 can be suitable forthe melt processable, water soluble polymer for this embodiment of theinvention.

[0049] The superabsorbent material can be any commercially availablesuperabsorbent material, such as superabsorbent particles orsuperabsorbent fibers. Examples of commercially available particulatesuperabsorbents include SANWET® IM 3900 and SANWET® IM-5000P, availablefrom Hoescht Celanese located in Portsmouth, Va., DRYTECH® 2035available from Dow Chemical Co. located in Midland, Mich., and FAVOR®SXM 880, SXM 9543, available from Stockhausen, located in Greensboro,N.C. Any of the previously described commercially availablesuperabsorbent staple fibers can be suitable superabsorbent fibers.

[0050] The hydrophilic fibers are preferably wood pulp fluffcommerically available from US Alliance Forest Products Corporation inCoosa Pine, Ala., USA, under the trade designation Coosa CR 1654.

[0051] The resulting absorbent composite, which is a coform material, asshown in FIGS. 2A-2C, includes the absorbent fibers 36 made in thisinstance from polyvinyl alcohol, hydrophilic fibers in this instancewood pulp fluff 38, and superabsorbent material 39 in this instancesuperabsorbent particles.

[0052] The invention also includes a method for making absorbent fibersand an absorbent composite. Referring to FIG. 3, a hopper 50 containspellets of a melt processable, water soluble polymer. A single or twinscrew extruder 52 melts the pellets by a conventional heatingarrangement to form a molten extrudable composition which is extrudedthrough a melt-blowing die 54 by the action of a turning extruder screw(not shown) located within the extruder 52. The extrudable compositionis fed through the die 54. The die 54 and the gas supply fedtherethrough are heated by a conventional arrangement (not shown).Besides the spinning die diameter, the air velocity can also be adjustedto control fiber diameter.

[0053] In order to produce a nonwoven material including only theabsorbent fibers of the invention, next, a solution containing thecross-linking agent is sprayed onto the gas borne stream of fibers 56 bya sprayer represented by stream 60. The absorbent fibers are thendirected onto a forming wire 64 including a belt 66 and rollers 68 byvacuum 67 to air form the nonwoven material which may then be dried andtreated by a post treatment process to initiate the cross-linkingreaction. Use of the vacuum box 67 underneath the forming wire 64 canhelp the fibers form a uniform web onto the forming wire 64. The posttreatment may be heat treatment, microwave treatment, e-beam radiation,UV radiation, steam treatment or vapor treatment. The nonwoven materialis then wound and collected onto a winder 70.

[0054] In order to produce the absorbent composite of the invention,which is a coform material, the gas borne stream of fibers 56 is mergedwith a secondary gas stream 58 containing individualized hydrophilicfibers, preferably wood pulp fibers, so as to integrate the differentfibrous materials in a single step. A solution containing thecross-linking agent is sprayed onto the gas borne stream of fibers 56 bya sprayer represented by stream 60. The superabsorbent material may beadded simultaneously with the hydrophilic fibers and cross-linking agentvia an additional gas stream 62. The integrated air stream is thendirected onto a forming wire 64 including a belt 66 and rollers 68 byvacuum 67 to air form the coform material. The air may be supplied byany conventional means as, for example, a blower (not shown).

[0055] Any of the previously described melt processable polymers,superabsorbent materials, hydrophilic fibers and cross-linking agentscan be used to make the absorbent fibers and/or absorbent composites.

[0056] Following formation of the coform material, the coform materialis dried and then treated by a post treatment process in order toinitiate the cross-linking reaction. Such post treatment is sometimesreferred to as “curing.” Such treatment may be any one of heattreatment, microwave treatment, e-beam radiation, UV radiationtreatment, steam treatment or vapor treatment. The coform material isthen wound and collected onto a winder 70.

[0057] When heat treatment is used as the post treatment process for thecoform material, undesirable discoloration of the coform materialsometimes occurs. For example, when a coform material including woodpulp fiber is heat cured at a temperature higher than 140° C. for morethan 2 hours, the cured coform material has a dark color ranging fromyellow to brown because of the oxidation of the wood pulp fiber. Suchdiscoloration will also occur when the meltblown fibers are made frompolyvinyl alcohol.

[0058] Effective ways to minimize or eliminate the discolorationinclude, but are not limited to: (1) reducing the curing temperature byusing catalysts or a low temperature curable cross-linking agent; (2)curing the coform material using a different curing method, such asmicrowave radiation or e-beam radiation; (3) using different types ofmelt processable polymers, such as hydroxy propyl cellulose; (4) using aself-cross-linkable polymer, such as silane grafted polyethylene oxide,which is capable of cross-linking itself induced by moisture; and (5)using an antioxidant.

[0059] During the preparation of the coform material, spraying waterduring the fiber spinning and in the superabsorbent material/wood pulpfluff/superabsorbent fiber mixing zone can help to enhance inter-fiberor inter-superabsorbent particle bonding. Therefore, the concentrationof the cross-linking agent can play a role in controlling the structureand integrity of absorbent composites. If more water is needed, a moredilute solution of cross-linking agent can be prepared, or vice versa.On the other hand, the concentration of the cross-linking agent affectsthe shell thickness of the surface of the cross-linked fiber with moredilute concentrations resulting in a thicker cross-linked surface shelllayer.

TEST METHOD—ABSORBENCY UNDER ZERO LOAD

[0060] The Absorbency Under Zero Load (AUZL) is a test which measuresthe ability of an absorbent material to absorb a liquid (such as a 0.9weight percent solution of sodium chloride in distilled water) whileunder a negligible load or restraining force. About 0.16 g of meltblownweb or coform discs (about 1 inch in diameter) of each sample wereweighed and placed into a plastic sample cup. The sample cup consists ofa plastic cylinder having a 1 inch inside diameter and an outsidediameter of 1.25 inches. The bottom of the sample cup is formed byadhering a 100 mesh metal screen having 150 micron openings to the endof the cylinder by heating the screen above the melting point of theplastic and pressing the plastic cylinder against the hot screen to meltthe plastic and bond the screen to the plastic cylinder. The sample isthen covered with a plastic spacer disc, weighing 4.4 grams, whichgenerated a pressure of about 0.01 pound per square inch. The sample cupis placed in a Petri dish which contains about 25 ml of 0.9% by weightsodium chloride solution. After one hour, the cup was taken out andplaced on multiple layers of paper towels to blot the interstitial fluidof the web or coform. The blotting is continued by moving the cup to thearea with dry paper towel until there is no fluid mark visible on thepaper towel. The weight difference of the cup between wet and drypresents total amount of fluid absorbed by the web or coform and is usedto calculate AUZL.

EXAMPLES Example 1

[0061] Water soluble polyvinyl alcohol Ecomaty® AX-10000, available fromNippon Gohsei, Osaka, Japan, was melt blown into continuous filamentsthrough a Killion line (2″ die tip, 0.35 mm die, 20 holes/in, otherparameters listed in Table 1). Air pressure was in a range from 4 to 8psi. Air temperature was controlled to as close to the die temperatureas possible (412° F. in this case) so that no effect of cooling off orheating up on die tip occurred. Vacuum was about 8 inch water. Thefilament was collected on a moving conveyor belt having a foraminoussurface to form a melt blown nonwoven. The nonwoven was still watersoluble and dipped into a solution of 86.5% methanol, 12.3% water, and1.2% ethylene glycol diglycidyl ether at a weight ratio of 1 g of fiberto 30 g of solution. The nonwoven was then removed out of the solutionand blotted by paper towel. The wet nonwoven was dried at 80° C. andthen cured at 130° C. for 20 hours. The cured nonwoven waswater-swellable but water-insoluble and exhibited an Absorbency UnderZero Load (AUZL) value in 0.9% NaCl saline of about 7.5 g/g. TABLE 1Heat 1 Barrel Die Extruder Temp. Heat 2 Temp. Heat 3 Temp. Die Temp.Pressure Pressure Screw (° F.) (° F.) (° F.) (° F.) (psi) (psi) Speed(rpm) 269 304 410 412 1020 340 11

Example 2

[0062] Several solutions including different cross-linking agents wereprepared as described in Table 2. The polyvinyl alcohol nonwovenprepared in Example 1 was treated separately by the solution and heatcured at different temperatures for certain times. The cured nonwovenswere subjected to the AUZL test in saline. Both the AUZL value and thecolor of the cured nonwoven were recorded in Table 2 below: TABLE 2Ratio of Curing Recipe # Composition Fiber/solution T(° C.)/time(hr)Discoloration AUZL (g/g) 1 98% methanol, 2% 1/50 130/15 Golden 5.5Kymene (557LX) 2 84.5% methanol, 14% 1/25 110/70 Light Golden 6.0 water,1.5% citric acid 3 88% methanol, 11.5% 1/15 140/20 Yellow 7.1 water,0.5% glutaric dialdehyde 4 85% methanol, 12.5% 1/30 130/24 White 8.2water, 2.5% ethylene glycol diglycidyl ether

Example 3

[0063] Polyvinyl alcohol was melt blown into continuous filaments usingthe Killion line. Solutions including 1.25%, 2.5% or 5% citric acid orKYMENE® were separately sprayed onto the surface of the fibers at alocation near the die tip. A coform material with wood pulp fluff CR1654 at a ratio of 50% polyvinyl alcohol and 50% wood pulp fluff wasalso prepared. The polyvinyl alcohol fiber spinning throughput was about30 grams per minute (gpm), and the solution spraying throughput wasabout 10 gpm. Process conditions are listed in Table 3. Air pressure wasin a range from 4 to 8 psi. Air temperature was controlled to as closeto the die temperature as possible so that no effect of cooling off orheating up on die tip occurred. Vacuum was about 8 inch water. Thenonwovens obtained were heated in a 130° C. oven for up to 4 days. Thecured nonwovens were completely cross-linked and become water insoluble.Again discoloration was found in all the cured nonwovens. AUZL values ofthe treated nonwovens were around 7 to 8 g/g. TABLE 3 Extruder Spin PumpDie Cross-linker Heater 1 Heater 2 Heater 3 Die Screw Speed SpeedPressure Solution (° F.) (° F.) (° F.) (° F.) (rpm) (rpm) (psi) 1.25%citric 306 361 440 440 25 20 396 acid  2.5% citric 306 341 440 442 24 20382 acid  5.0% citric 303 342 440 438 28 20 390 acid 1.25% 306 360 440439 21 20 402 Kymene 2.5% 305 360 440 440 21 19.7 408 Kymene 5.0% 305360 440 440 22 20 394 Kymene

Example 4

[0064] The uncured nonwovens surface sprayed by either 5% citric acid orKYMENE® solution and prepared from Example 3 were treated in a microwaveoven (GE Model JE125OGW, 1.5 kW, Vac/Hz 120/60) at an intensity level of1 (lowest of the machine) for at least 10 hours. The intensity level ofthe microwave oven could not be greater than 1, otherwise the nonwovenfibers would be molten due to too high temperature reached locally. Themicrowave treated nonwovens were completely white (no discoloration) andwater-swellable, water-insoluble.

Example 5

[0065] Polyvinyl alcohol was melt blown into continuous filaments usingthe Killion line. A solution including 5% KYMENE® and 0.5% surfactantRhodamox LO, available from Rhone-Poulenc Inc., was sprayed onto thesurface of fibers at a location near the die tip. A coform material withboth commercial superabsorbent particles FAVOR® SXM 880 and wood pulpfluff CR 1654 at a ratio of 48% superabsorbent particles, 26% polyvinylalcohol and 26% wood pulp fluff was also prepared. Process parametersare listed in Table 4. Air pressure, temperature and vacuum were thesame as specified in the examples before. The basis weight of the coformmaterial was 484 gsm. A solution including 5% KYMENE® and 0.5%surfactant Rhodamox LO was sprayed onto the surface of the coformmaterial at a location near the tie tip. The coform material was heatcured at 150 ° C. for 3 hours. Surprisingly, the cured coform materialhad almost no discoloration probably due to the presence of Rhodamox LOsurfactant. The coform material exhibited an AUZL value in 0.9% NaClsaline as high as 23 g/g. TABLE 4 Extruder Heater 1 Heater 2 Heater 3Die Die Pressure Screw Speed Spin Pump (° F.) (° F.) (° F.) (° F.) (psi)(rpm) Speed (rpm) 321 401 440 443 900 27 20

[0066] While the embodiments of the invention described herein arepresently preferred, various modifications and improvements can be madewithout departing from the spirit and scope of the invention. The scopeof the invention is indicated by the appended claims, and all changesthat fall within the meaning and range of equivalents are intended to beembraced therein.

We claim:
 1. An absorbent fiber, comprising: a melt processable, watersoluble polymer; and a cross-linking agent; wherein the absorbent fiberhas an absorbency under zero load of at least about 5 g/g.
 2. Theabsorbent fiber of claim 1, wherein the polymer comprises a polymerselected from polyethylene oxide, polypropylene oxide, hydroxyl propylcellulose, methyl cellulose, ethyl cellulose, methylethyl cellulose,polyethylene imine, polyvinyl amine, polyvinyl alcohol, poly(ethyleneoxide-co-propylene oxide), polyacrylic acid, polyacrylamide, andcombinations thereof.
 3. The absorbent fiber of claim 1, wherein thepolymer comprises a copolymer.
 4. The absorbent fiber of claim 3,wherein at least one monomer of the copolymer is sodium acrylate.
 5. Theabsorbent fiber of claim 3, wherein at least one monomer of thecopolymer is methyl methacrylate.
 6. The absorbent fiber of claim 3,wherein the copolymer includes a ratio on a dry weight basis of a firstmonomer to a second monomer of from about 30:70 to about 70:30.
 7. Theabsorbent fiber of claim 3, wherein the copolymer comprises sodiumacrylate and methyl methacrylate.
 8. The absorbent fiber of claim 1,wherein the polymer has a molecular weight in a range of about 10,000 toabout 1,000,000.
 9. The absorbent fiber of claim 1, wherein the polymerhas a molecular weight in a range of about 50,000 to about 1,000,000.10. The absorbent fiber of claim 1, wherein the polymer has a molecularweight in a range of about 100,000 to about 500,000.
 11. The absorbentfiber of claim 1, wherein the cross-linking agent comprises at least twofunctional groups capable of reacting with functional groups on thepolymer.
 12. The absorbent fiber of claim 11, wherein the at least twofunctional groups of the cross-linking agent comprise a functional groupselected from carboxylic acid group, epoxy group, hydroxyl group, aminogroup, aldehyde group, tri-valent metal ions, and tetra-valent metalions.
 13. The absorbent fiber of claim 1, wherein the cross-linkingagent comprises a compound selected from diols, polyols, diamines,polyamines, dicarboxylic acids, polycarboxylic acids, dialdehydes,polyaldehydes, butandiol, diethylene triamine, ethylene glycoldiglycidyl ether, citric acid, glutaric dialdehyde, and combinationsthereof.
 14. The superabsorbent fiber of claim 1, wherein across-linking reaction is initiated by a treatment selected from heattreatment, microwave radiation, e-beam radiation, UV radiation, steamtreatment, and vapor treatment.
 15. An absorbent composite, comprising:a melt processable, water soluble polymer; a superabsorbent material;hydrophilic fibers; and a cross-linking agent; wherein the absorbentcomposite has an absorbency under zero load of at least about 10 g/g.16. The absorbent composite of claim 15, wherein the superabsorbentmaterial comprises superabsorbent particles.
 17. The absorbent compositeof claim 15, wherein the superabsorbent material comprisessuperabsorbent fibers.
 18. The absorbent composite of claim 15, whereinthe hydrophilic fibers comprise fibers selected from wood pulp fluff,cotton, cotton linter, other cellulose fibers, regenerated cellulosefibers, staple fibers, synthetic hydrophilic fibers, and combinationsthereof.
 19. The absorbent composite of claim 15, wherein the polymercomprises a polymer selected from polyethylene oxide, polypropyleneoxide, hydroxyl propyl cellulose, methyl cellulose, ethyl cellulose,methylethyl cellulose, polyethylene imine, polyvinyl amine, polyvinylalcohol, poly(ethylene oxide-co-propylene oxide), polyacrylic acid,polyacrylamide, and combinations thereof.
 20. The absorbent composite ofclaim 15, wherein the polymer comprises a copolymer.
 21. The absorbentcomposite of claim 20, wherein at least one monomer of the copolymer issodium acrylate.
 22. The absorbent composite of claim 20, wherein atleast one monomer of the copolymer is methyl methacrylate.
 23. Theabsorbent composite of claim 20, wherein the copolymer includes a ratioon a dry weight basis of a first monomer to a second monomer of fromabout 30:70 to about 70:30.
 24. The absorbent composite of claim 20,wherein the copolymer comprises sodium acrylate and methyl methacrylate.25. The absorbent composite of claim 15, wherein the polymer has amolecular weight in a range of about 10,000 to about 1,000,000.
 26. Theabsorbent composite of claim 15, wherein the polymer has a molecularweight in a range of about 50,000 to about 1,000,000.
 27. The absorbentcomposite of claim 15, wherein the polymer has a molecular weight in arange of about 100,000 to about 500,000.
 28. The absorbent composite ofclaim 15, wherein the cross-linking agent comprises at least twofunctional groups capable of reacting with functional groups on thepolymer.
 29. The absorbent composite of claim 28, wherein the at leasttwo functional groups of the cross-linking agent comprise a functionalgroup selected from carboxylic acid group, epoxy group, hydroxyl group,amino group, aldehyde group, tri-valent metal ions, and tetra-valentmetal ions.
 30. The absorbent composite of claim 15, wherein thecross-linking agent comprises a compound selected from diols, polyols,diamines, polyamines, dicarboxylic acids, polycarboxylic acids,dialdehydes, polyaldehydes, butandiol, diethylene triamine, ethyleneglycol diglycidayl ether, citric acid, glutaric dialdehyde, andcombinations thereof.
 31. The absorbent composite of claim 15, wherein across-linking reaction is initiated by a treatment selected from heattreatment, microwave, e-beam radiation, UV radiation, steam treatment,and vapor treatment.
 32. The absorbent composite of claim 15, whereinthe absorbent composite is a superabsorbent material.
 33. The absorbentcomposite of claim 15, wherein the absorbent composite has an absorbencyunder zero load of at least 15 g/g.
 34. The absorbent composite of claim15, wherein the absorbent composite has an absorbency under zero load ofup to about 50 g/g.
 35. A method of producing an absorbent fiber,comprising the steps of: melting a melt processable, water solublepolymer; extruding the polymer; spinning the polymer to form fibers;adding a cross-linking agent to the fibers; and curing the fibers. 36.The method of claim 35, wherein the polymer comprises a polymer selectedfrom polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose,methyl cellulose, ethyl cellulose, methyethyl cellulose, polyethyleneimine, polyvinyl amine, polyvinyl alcohol, poly(ethyleneoxide-co-propylene oxide), polyacrylic acid, polyacrylamide, andcombinations thereof.
 37. The method of claim 35, wherein the polymercomprises a copolymer.
 38. The method claim 37, wherein at least onemonomer of the copolymer is sodium acrylate.
 39. The method of claim 37,wherein at least one monomer of the copolymer is methyl methacrylate.40. The method of claim 37, wherein the copolymer includes a ratio on adry weight basis of a first monomer to a second monomer of from about30:70 to about 70:30.
 41. The method of claim 37, wherein the copolymercomprises sodium polyacrylate and methyl methacrylate.
 42. The method ofclaim 35, wherein the polymer has a molecular weight in a range of about10,000 to about 1,000,000.
 43. The method of claim 35, wherein thepolymer has a molecular weight in a range of about 50,000 to about1,000,000.
 44. The method of claim 35, wherein the polymer has amolecular weight in a range of about 100,000 to about 500,000.
 45. Themethod of claim 35, wherein the cross-linking agent comprises at leasttwo functional groups capable of reacting with functional groups on thepolymer.
 46. The method of claim 45, wherein the at least two functionalgroups of the cross-linking agent comprise a functional group selectedfrom carboxylic acid group, epoxy group, hydroxyl group, amino group,aldehyde group, tri-valent metal ions, and tetra-valent metal ions. 47.The method of claim 35, wherein the cross-linking agent comprises acompound selected from diols, polyols, diamines, polyamines,dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes,butandiol, diethylene triamine, ethylene glycol diglycidyl ether, citricacid, glutaric dialdehyde, and combinations thereof.
 48. The method ofclaim 35, wherein a cross-linking reaction is initiated by a treatmentselected from heat treatment, microwave, e-beam radiation, UV radiation,steam treatment, and vapor treatment.
 49. A diaper comprising theabsorbent fiber produced according to the method of claim
 35. 50.Training pants comprising the absorbent fiber produced according to themethod of claim
 35. 51. Swim wear comprising the absorbent fiberproduced according to the method of claim
 35. 52. An adult incontinencegarment comprising the absorbent fiber produced according to the methodof claim
 35. 53. A feminine hygiene product comprising the absorbentfiber produced according to the method of claim
 35. 54. A medicalabsorbent product comprising the absorbent fiber produced according tothe method of claim
 35. 55. A method of producing an absorbentcomposite, comprising the steps of: melting a melt processable, watersoluble polymer; extruding the polymer; spinning the polymer to formfibers; adding hydrophilic fibers to the fibers; adding a superabsorbentmaterial to the fibers; adding a cross-linking agent to the fibers; andcuring the fibers.
 56. The method of claim 55, wherein the hydrophilicfibers comprise fibers selected from wood pulp fluff, cotton, cottonlinter, other cellulose fibers, regenerated cellulose fibers, staplefibers, synthetic hydrophilic fibers, and combinations thereof.
 57. Themethod of claim 55, wherein the polymer comprises a polymer selectedfrom polyethylene oxide, polypropylene oxide, hydroxyl propyl cellulose,methyl cellulose, ethyl cellulose, methylethyl cellulose, polyethyleneimine, polyvinyl amine, polyvinyl alcohol, poly(ethyleneoxide-co-propylene oxide), polyacrylic acid, polyacrylamide, andcombinations thereof.
 58. The method of claim 55, wherein the polymercomprises a copolymer.
 59. The method claim of 58, wherein at least onemonomer of the copolymer is sodium acrylate.
 60. The method of claim 58,wherein at least one monomer of the copolymer is methyl methacrylate.61. The method of claim 58, wherein the copolymer includes a ratio on adry weight basis of a first monomer to a second monomer of from about30:70 to about 70:30.
 62. The method of claim 58, wherein the copolymercomprises sodium acrylate and methyl methacrylate.
 63. The method ofclaim 55, wherein the polymer has a molecular weight in a range of about10,000 to about 1,000,000.
 64. The method of claim 55, wherein thepolymer has a molecular weight in a range of about 50,000 to about1,000,000.
 65. The method of claim 55, wherein the polymer has amolecular weight in a range of about 100,000 to 500,000.
 66. The methodof claim 55, wherein the cross-linking agent comprises at least twofunctional groups capable of reacting with functional groups on thepolymer.
 67. The method of claim 66, wherein the at least two functionalgroups of the cross-linking agent comprise a functional group selectedfrom carboxylic acid group, epoxy group, hydroxyl group, amino group,aldehyde group, tri-valent metal ions, and tetra-valent metal ions. 68.The method of claim 55, wherein the cross-linking agent comprises acompound selected from diols, polyols, diamines, polyamines,dicarboxylic acids, polycarboxylic acids, dialdehydes, polyaldehydes,butandiol, diethylene triamine, ethylene glycol diglycidyl ether, citricacid, glutaric dialdehyde, and combinations thereof.
 69. The method ofclaim 55, wherein a cross-linking reaction is initiated by a treatmentselected from heat treatment, microwave, e-beam radiation, UV radiation,steam treatment, and vapor treatment.
 70. A diaper comprising theabsorbent composite produced according to the method of claim
 55. 71.Training pants comprising the absorbent composite produced according tothe method of claim
 55. 72. Swim wear comprising the absorbent compositeproduced according to the method of claim
 55. 73. An adult incontinencegarment comprising the absorbent composite produced according to themethod of claim
 55. 74. A feminine hygiene product comprising theabsorbent composite produced according to the method of claim
 55. 75. Amedical absorbent product comprising the absorbent composite producedaccording to the method of claim 55.